The invention relates to a community antenna arrangement for a frequency-division multiplex transmission of at least four television signals, comprising a frequency-conversion arrangement, connected to a distribution network, for converting the frequency distribution of the television signals from an input channel frequency distribution pattern, hereinafter referred to as channel pattern, into an output channel pattern. In the output channel pattern the picture carriers of the television signals succeed each other with frequency spacings of which not more than two consecutive frequency spacings are mutually equal.
Such a community antenna arrangement is disclosed in German Offenlegungsschrift No. 2.738.217.
Generally, in the distribution network of a community antenna arrangement for frequency-division multiplex transmission of TV signals, one or more broadband signal processing operations are effected. Several channels are submitted, as one frequency-division multiplex signal, to the same operation. Such broadband signal processing operations may, for example, be: block conversion in which the frequency of several channels are jointly converted, broadband amplification, broadband electro-optical or opto-electrical conversions, etc. Non-linearities cannot be prevented from occurring in practice, even if circuits having highly linearized frequency characteristics for these broadband processing operations are used.
These non-linearities produce intermodulation products which, depending on the number, frequency and amplitude of the radio and/or television signals to be transmitted, cause disturbances to a greater or lesser extent. Particularly, the intermodulation products of the carriers, and more specifically, the intermodulation products of the picture carriers of the TV-signals to be transmitted are important in this respect, because of their comparatively large amplitude.
The disturbing effect of second order intermodulation products can be reduced in several ways, for example, by using balanced circuits, such as push-pull amplifiers or by limitation of the frequency range in which the above-mentioned broadband non-linear signal processings are operative to a maximum of 1 octave, such as occurs, for example, with VHF/UHF split-band amplification. With said, prior art community antenna arrangement, the object is to reduce unwanted second order interferring products, caused by VHF-signals in combination with UHF-signals, by using a certain channel pattern, such that said interferring products occur outside the channels or at less critical places within the channels.
In the prior art community antenna arrangement, measures are not taken to suppress the interferences due to 3rd and higher order intermodulation products. Generally, the amplitude of the 3.sup.rd and higher order intermodulation products are smaller than the amplitude of the second order intermodulation products, for which reason little attention is paid to their suppression in the prior art community antenna arrangement.
However, the increasing demand for ever more TV programs causes an increasing occupation of the available frequency space in the distribution network of the majority of community antenna arrangements. The number of higher order intermodulation products compared with the number of second order intermodulation products, increases significantly with the number of TV programs to be transmitted. This increase in the number of higher order intermodulation products is so great that when a large number of TV programs (for example 30 or more) are to be transmitted, the interference aspect is predominantly determined by these higher order intermodulation products.